With the advent of the Tesla Roadster, a new generation of 100% battery powered car is upon us. It’s about time. But hybrids with extra battery packs, known as “strong hybrids,” and their counterparts, hybrids that you can plug in to recharge, appropriately known as “plug-in hybrids,” are moving out of the hands of tinkerers and into the mainstream.
An interesting study from CalCars.org, entitled “100+ MPG Hybrids” makes the case that cars powered by electricity from the power grid can get over 100 miles to the gallon. This is somewhat misleading – because cars powered from the grid, to the extent they’re using grid electricity stored on-board, are not getting miles per gallon, they’re getting miles per kilowatt-hour.
Assume a car is a strong hybrid, with range on a battery charge sufficient to fulfill a normal daily commute cycle, and assume that the car is a plug-in hybrid, getting recharged at night from the power grid so no gasoline energy whatsoever is used in its daily functions. This is not a huge assumption – these cars are here today, and soon they will make it out of the tinkerer’s garages and onto the dealer’s showroom floors. How much per mile does it cost to drive these cars?
The math isn’t all that challenging if you are really interested in knowing the answer. First of all, assume a gas powered car gets 30 MPG, and gas costs $3.00. This means a gas car costs $.10 per mile to drive.
Next assume a gasoline powered car has an engine that converts the energy in gasoline into mechanical energy at an efficiency of 25%. This is typical; the rest of the energy is lost in extraneous motion, friction and heat. This means that if a gasoline engine were 100% efficient, that same car could go 120 miles on a gallon instead of only 30 miles per gallon.
Here’s where it gets interesting. A battery will recharge and discharge kilowatt-hours from the power grid at an efficiency of 90%. An electric motor will convert electricity into mechanical energy at an efficiency that is also about 90% (the larger the engine the better the efficiency). This means a battery powered electric car will convert kilowatt-hours from the power grid into mechanical energy at an efficiency of over 80% (90% times 90%).
For this reason, a battery powered car can take that same one gallon of gasoline, using the equivalent amount in kilowatt-hours, and go 96 miles, more than three times what a gasoline powered car can do.
The rest is simple. There are 32.91 kilowatt-hours of energy in a gallon of gas, and the market cost consumers pay for kilowatt-hours is about $.10 (this varies widely, but for recharging at night during off-peak rates $.10 is probably on the high side), which means for $3.29 you can drive an electric car 96 miles. That equates to 2.9 miles per kilowatt-hour, or 3.4 cents per mile. Compared to gasoline powered cars, all-electric cars use far less energy to drive the same distance, and consequently cost far less to fuel.
This is why we will see strong hybrids, plug-in hybrids, and 100% battery powered cars on the roads within a few short years. For references and more in-depth explanations of these formulas, read “The Battery Powered Car.”